Optimized printing process enables custom organic electronics

2015-06-17 – News from Physik-Department

They are thin, light-weight, flexible and can be produced cost- and energy-efficiently:
printed microelectronic components made of synthetics.
Flexible displays and touch screens, glowing films, RFID tags and solar cells represent a future market.
In the context of an international cooperation project,
physicists at the physics department of TUM have now observed the creation of
razor thin polymer electrodes during the printing process and successfully
improved the electrical properties of the printed films.

Solar cells out of a printer? This seemed unthinkable only a few years
ago. There were hardly any alternatives to classical silicon technology
available. In the mean time touch screens, sensors and solar cells can
be made of conducting synthetics. Flexible monitors and glowing wall
paper made of organic light emitting diodes, so-called OLEDs, are in
rapid development. The “organic electronics” are hailed as a promising
future market.

However, the technology also has its pitfalls: To manufacture the
components on an industrial scale, semiconducting or insulating layers –
each a thousand times thinner than a human hair – must be printed onto a
carrier film in a predefined order. “This is a highly complex process,
whose details need to be fully understood to allow custom-tailored
applications,” explains Professor Peter Müller-Buschbaum of the Chair of
Functional Materials at TU München.

A further challenge is the contacting between flexible, conducting
layers. Hitherto electronic contacts made of crystalline indium tin
oxide were frequently used. However, this construction has numerous
drawbacks: The oxide is more brittle than the polymer layers over them,
which limits the flexibility of the cells. Furthermore, the
manufacturing process also consumes much energy. Finally, indium is a
rare element that exists only in very limited quantities.

Polymers in X-ray light

A few months ago, researchers from the Lawrence Berkeley National
Laboratory in California for the first time succeeded in observing the
cross-linking of polymer molecules in the active layer of an organic
solar cell during the printing process. In collaboration with their
colleagues in California, Müller-Buschbaum’s team took advantage of this
technology to improve the characteristics of the polymer electronic
elements.

The researchers used X-ray radiation generated in the Berkley
synchrotron for their investigations. The X-rays are directed to the
freshly printed synthetic layer and scattered. The arrangement and
orientation of the molecules during the curing process of the printed
films can be determined from changes in the scattering pattern.

“Thanks to the very intensive X-ray radiation we can achieve a very high
time resolution,” says Claudia M. Palumbiny. In Berkeley the physicist
from the TUM investigated the “blocking layer” that sorts and
selectively transports the charge carriers in the organic electronic
components. The TUM research team is now, together with its US
colleagues, publishing the results in the trade journal Advanced
Materials.

Custom properties

“In our work, we showed for the first time ever that even small changes
in the physico-chemical process conditions have a significant influence
on the build-up and properties of the layer,” says Claudia M. Palumbiny.
“Adding solvents with a high boiling point, for example, improves
segregation in synthetics components. This improves the crystallization
in conducting molecules. The distance between the molecules shrinks and
the conductivity increases.

In this manner stability and conductivity can be improved to such an
extent that the material can be deployed not only as a blocking layer,
but even as a transparent, electrical contact. This can be used to
replace the brittle indium tin oxide layers. “At the end of the day,
this means that all layers could be produced using the same process,”
explains Palumbiny. “That would be a great advantage for manufacturers.”

To make all of this possible one day, TUM researchers want to continue
investigating and optimizing the electrode material further and make
their know-how available to industry. “We have now formed the basis for
pushing ahead materials development with future investigations so that
these can be taken over by industrial enterprises,” explains Prof.
Müller-Buschbaum.

The research was supported by the GreenTech Initiative “Interface
Science for Photovoltaics” (ISPV) of the EuroTech Universities together
with the International Graduate School of Science and Engineering
(IGSSE) at TUM and by the Cluster of Excellence “Nanosystems Initiative
Munich” (NIM). Further support came from the Elite Network of Bavaria’s
International Doctorate Program “NanoBioTechnology” (IDK-NBT) and the
Center for NanoScience (CeNS) and from “Polymer-Based Materials for
Harvesting Solar Energy” (PHaSE), an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Basic Energy
Sciences. Portions of the research were carried out at the Advanced
Light Source which receives support by the Office of Basic Energy
Sciences of the U.S. Department of Energy.